A cargo ship transporting crude oil or the like has ballast tanks for maintaining the stability of a hull during navigation. Usually, the ballast tanks are filled with ballast water when the ship is not loaded with crude oil or the like, and the ballast water is discharged when crude oil or the like is loaded into the ship. In this way, the buoyancy of the hull is adjusted to stabilize the hull. As described above, the ballast water is water required for safe navigation of the ship, and normally seawater at a port where cargo handling is carried out is used. The amount thereof is estimated to be over 10 billion tons per year on a worldwide basis.
Incidentally, the ballast water contains microorganisms or eggs of small and large organisms, which inhabit port where the ballast water was taken in. As the ship moves, such microorganisms or eggs of small and large organisms are also transported to foreign countries. Therefore, the destruction of the ecosystem caused as a result of organism species which do not originally inhabit a sea area replacing existing organism species is getting serious.
Against this background, a diplomatic conference of the International Maritime Organization (IMO) has adopted an obligation to conduct regular inspections on ballast water treatment equipment and the like, and this obligation is applied to ships constructed after 2009.
Moreover, the discharge standard (G8) set forth in Regulation D-2 of the Convention for the Control and Management of Ships' Ballast Water and Sediments (hereinafter referred to as the Convention) is as follows: plankton with a size of 10 to 50 μm must be 10 cells/ml, plankton with a size of 50 μm or more must be 10 cells/m3, Escherichia coli with a size of 0.5 to 3 μm must be 250 cfu/100 ml, Vibrio cholerae with a size of 0.5 to 3 μm must be 1 cfu/100 ml, and Enterococcus with a size of 0.5 to 3 μm must be 100 cfu/100 ml.
Furthermore, according to the Procedure for approval of ballast water management systems that make use of active substances (G9), which was adopted on Jul. 22, 2005, the objective of the Procedure is to determine the acceptability of active substances and preparations containing one or more active substances and their application in ballast water management systems concerning ship safety, human health, and the aquatic environment. According to G9, an active substance means a substance or organism, including a virus or a fungus that has a general or specific action on or against harmful aquatic organisms and pathogens, and the Procedure requires toxicity testing for verification.
Against this backdrop, extremely strict sterilization or bacterial eradication is required to meet the requirements of G8 and G9.
As the ballast water sterilization/bacterial eradication technology, a technique described in Patent Document 1 is conventionally known as an ozone-based chemical method.
Patent Document 1 proposes a technique of performing sterilization by injecting ozone into ballast water in conjunction with an injection of steam while reducing ozone usage by turning the ozone into microscopic bubbles and thereby promoting the generation of hydroxyl radicals.
However, the reduction of ozone usage has its limit. As a result, research is rapidly going on to adopt membrane treatment.
Many gel-like substances of biological origin exist in ballast water (seawater), and, when the ballast water is subjected to membrane treatment by using a membrane module, these substances may accumulate on the membrane surface in prolonged operation. The problem is that these gel-like substances firmly attach to the membrane surface, and are difficult to remove even when the membrane is backwashed.
Therefore, in the membrane treatment method, anti-fouling measures become important to prolong the membrane treatment time. As the anti-fouling measures taken at the time of membrane treatment, in Patent Document 2, a technique of performing membrane cleaning by using an enzyme and a technique of performing membrane cleaning by using an enzyme and an oxidizer in combination are disclosed. In Patent Document 3, a technique of performing membrane cleaning by using ethylenediaminetetraacetic acid tetrasodium tetrahydrate is disclosed.
Moreover, as a method in which membrane treatment is not performed, a method in which treatment is performed with iodine is disclosed in Patent Document 4, and a method in which treatment is performed with sodium hypochlorite is disclosed in Patent Document 5.                Patent Document 1: JP-A-2004-160437        Patent Document 2: JP-A-3-133947        Patent Document 3: JP-A-11-319518        Patent Document 4: JP-T-2002-504851        Patent Document 5: JP-A-4-322788        